Hollow plastic products such as some fuel tanks and similar products, have been made by an extrusion and blow molding process. In this process, a molten plastic parison is formed by extrusion and then transferred to a blow molding machine wherein a pressurized fluid is introduced into the interior of the parison within the mold to force the molten parison into engagement with the interior walls of the mold. When cooled and solidified, the molded plastic part will have an exterior shape corresponding to the interior of the mold.
After extrusion and before being blow molded, the parison is hollow and has a generally cylindrical or tubular shape. In some applications, such as the production of automotive fuel tanks, the parison is large and may have a substantial length on the order of eight feet or longer. During transfer of the parison from the extruder to the blow molding machine it is imperative to prevent the collapse of the parison wall upon itself to maintain the hollow center of the parison in which the pressurized fluid is received to expand the parison within the mold.
Plastic products formed from prior blow molding processes have had non-uniform wall thicknesses with corners or other sharply contoured areas, and especially upper corners or contoured areas in the upper portion of the molded product, having substantially thinner walls compared to other portions of the molded product. Thus, to ensure that these contoured portions and especially the upper corner areas of a molded product have at least a necessary minimum wall thickness, the other portions of the product must be formed with walls which are thicker than necessary thereby substantially increasing the amount of material and the cost to manufacture the product.
One device used to transfer a molten parison from an extruder to a mold is disclosed in Canadian Patent No. 2,135,544. In this '544 patent a parison carrying device has a collar which engages the parison after it emerges from the extruder and transfers the parison to a mold without pinching or collapsing of the parison walls. The collar has first and second halves which are made to grip the parison and which have fingers which are either fixed and hence, not movable, or which may pivot about a fixed axis to enable the collar to pinch and close one end of the parison to permit an increased pressure to be developed within the interior cavity of the parison. These fingers whether fixed or pivoted about a fixed axis, do not provide any stretching or redistribution of the parison material and therefore, plastic products formed with use of this parison carrying device have the aforementioned problem of non-uniform wall thickness with upper corners and other contoured portions of the product formed with thinner walls than other portions of the product.